About this Author

College chemistry, 1983

The 2002 Model

After 10 years of blogging. . .

Derek Lowe, an Arkansan by birth, got his BA from Hendrix College and his PhD in organic chemistry from Duke before spending time in Germany on a Humboldt Fellowship on his post-doc. He's worked for several major pharmaceutical companies since 1989 on drug discovery projects against schizophrenia, Alzheimer's, diabetes, osteoporosis and other diseases.
To contact Derek email him directly: derekb.lowe@gmail.com
Twitter: Dereklowe

August 23, 2011

SRT1720: Good (And Confusing) News for Obese Mice

Posted by Derek

Readers of this blog will be fairly familiar with the long, interesting story of sirtuin activators. Today we will speak of SRT1720, of which we have spoken before. This molecule was described in 2007 as an activator of Sirt1 with beneficial effects in rodent models of diabetes. But both of those statements were called into question by a series of papers which found difficulties with both the in vitro and the in vivo results (summarized here). The GSK/Sirtris team fired back, but that paper also served as a white flag on the in vitro assay questions: there were indeed artifacts due to the fluorescent peptides used. (Another paper has since confirmed these problems and proposed an off-target mechanism).

But that GSK response didn't address the in vivo assay questions at all - we still had a situation where one group said that these compounds (SRT1720 in particular) were beneficial, and another said that it showed no benefit and was toxic at higher doses. Adding to the controversy, another paper appeared late last year that went back to nematodes, and found the SRT1720 did not extend their lives, either. The state of this field can be fairly described, then, as "extremely confused".

Now we have a new paper whose title gets right down to it: "SRT1720 improves survival and healthspan of obese mice". First time I've seen "healthspan" as a word, I might add, and another interesting sidelight is that this appears in Nature Scientific Reports, the publishing group's open-access experiment. But now to the data:

What this (large) team did was place one-year-old male mice on a high-fat diet in the presence of two different doses of SRT1720 in the chow, corresponding to 30 mg/kilo and 100mg/kilo. The effects on lifespan were notable: standard-diet animals had a median lifespan of 125 weeks, and that was shortened to 94 weeks on the high fat diet. But on that diet plus the lower dose of SRT1720, the median lifespan was 103 weeks, and on the higher dose it was 115 weeks. It's interesting, though, that this took place while the animals ate the same number of calories and gained the same amount of (extra) weight as the control group.

Blood work and histopathology revealed many more differences. The high-fat animals (with no SRT1720) showed the expected problems that you see in such studies - fat accumulation in the liver, increased numbers of beta-cells in the pancreas, higher insulin levels, and so on. But the SRT1720-dosed animals showed a good deal of reversal of all these effects. DIgging down to the molecular level, inflammatory markers, indicators of apoptosis and DNA fragmentation were increased in the high-fat animals, and these were also mitigated by SRT1720.

There are many other effects mentioned in the paper, but I'm not going to go into all the details - hey, it's open-access, so if you're really into this stuff you can find it all. Suffice it to say that a long list of deleterious effects of a high-fat diet on rodents seem to be partially to fully reversed on treatment with SRT1720, particularly at the higher dose, without significant evidence of toxicity. But how do we reconcile that with the report that the compound showed no benefit, and toxic effects to boot? I'll let the authors tackle that one:

Our results continue to support the beneficial pharmacological effect of SRT1720 in models of metabolic disease despite a recent report by Pacholec and colleagues to the contrary14 where the authors report 100 mg/kg SRT1720 is not tolerable and increases mortality in mice and that the compound does not elicit beneficial effects in the Lep ob/ob mouse model of diabetes. This conclusion is inconsistent with not only our findings but also several additional studies where SRT1720 has been reported to exert positive effects in multiple models of metabolic disease including Lep ob/ob mice, diet-induced obese mice, MSG-induced hypothalamic obese mice15 and Zucker fa/fa rats. Pacholec and colleagues did report that fasting insulin levels are reduced by SRT1720 administration, which is in agreement with our findings (Fig. 2) and with data reported previously in diet-induced obese mice. The putative toxicity of SRT1720 administered at a 100 mg/kg oral dose to 8 mice over 18 days is inconsistent with a study where the compound exhibited no toxicity at a 5-fold higher dose for 15 weeks12 nor is it consistent with our long-term feeding study involving over 100 mice consuming an equivalent daily dose. In fact, our mice showed increased survival and improvement in multiple physiological parameters in response to SRT1720 treatment and did not display overt signs of toxicity even after more than 80 weeks of treatment.

So yes, there's pretty much a flat contradiction here, and I have no idea of how to resolve it. This paper doesn't reference the failure of SRT1720 to show effects in nematodes, but that's another piece of the puzzle that can't be ignored, either. One possibility is that the doses of the compound need to be rather heroic. Believe me, by the usual pharmacological standards, extended dosing at 100 mpk is pretty heavy-duty (and, I might add, basically unattainable in humans under normal conditions, especially humans on a high-fat diet).

So for now, I have to throw up my hands. This latest paper seems very thorough, and represents a really significant effort on the part of a long list of highly competent people. But there can be no doubt that the SRT1720 story (and the story of sirtuin activators in general) is still very complex and hard to evaluate, because the various problems and complications that have been found can't be dismissed, either. There's something here, all right, and it could well be very important. But what are we looking at?

Side note: this work was the subject of a writeup by Nicholas Wade in the New York Times the other day. It reveals that there's another arm of this study - normal mice, on normal chow, also treated with SRT1720. Those results, out next year, will be very interesting indeed, although I can only think that they're just going to keep the fires burning. I'd also like to note (as one comment on this blog did) the tone of most of the online comments on the Times story. They can, I think, be summed up as "Great, the big evil drug companies have found something so people can just stay big and fat and not die early, and they're going to sell it to us for a zillion dollars while their corporate masters stay thin and healthy and laugh at us all". Read through a few of them and see if I haven't captured their general spirit - and think for a bit about what that tells us, both about the public perception of drug research and (perhaps) about the sort of people who leave comments over at the Times.

I won't pretend I know the subject at all, but I as far as differences in results while using the same TA doses, I can't help but notice that this study uses old mice with a TA administered in the feed, while the preceding one used young mice with oral-gavage treated mice. These methodological differences alone (old vs young, ad lib vs gavage) could be enough to lead to significantly different results.

By the way, reading web comments sections for 5 minutes is enough to make you lose all faith (or even mere tolerance) for humanity - it should be avoided at all costs (present company excepted, or course).

Very similar to a study by Feige et al in Cell Metabolism 8, 347 (2008). Although these authors state that their results support those reported previously by Sirtris, they only saw favorable metabolic effects at doses that caused weight loss, and only when dosing chronically. Given the high doses used (they saw no effect against most endpoints at doses less than 500 mg/kg) and the fact that drug was admixed with chow, most of these results seem compatible with the drugs effects being mainly due to taste-aversion induced reduction in food uptake and consequent weight loss.

Taste aversion? Maybe, but at extremely high and higher doses I would bet that the funky thiadiazo heterocycle is quenching free radicals or being bioreduced like nobody's business. AND we all know what HAPPENS then.

I wonder how many targets this molecule actually hits at high concentrations?

At least it is'nt a resveratrol derivative and at 100 mgk they should have just rolled the animals in it, although the dose would have smaller.

Do I smell a promiscuous inhibitor? Only my dynamic light scattering machine knows for sure!

The methods in this paper are inadequate to know what was done in the histopath, and therefore, I wouldn't put much weight on that. I also don't see a veterinary pathologist as a co-author, which would give it credibility in my eyes. I only see references to pancreas and liver for histopathology, but the methods seem to imply that more extensive tissue evaluation was done. Also, it is not clear that the liver was assessed on histopathology for anything more than fat content via oil-red-O. Do we truly know why there was a difference in survival without full histologic assessment of many tissues? They also refer to AST and ALT activities as being liver function tests - these don't assess liver function, they assess liver injury. Not sure that the studies were as tightly run as they could be. I'll let others debate the biochemical assays.

John #2 is exactly right. The mice on the high concentration of drug weighed about 10% less than the controls throughout most of the HFD (look at Figure 1b). That's the whole effect, and it's probably just that the mice didn't like the taste of the drug.

The focus on "healthspan" and the use of lifespan as an endpoint are just misdirection meant to link the paper to the crazy Sirtuin-Aging narrative. But that's obviously misleading. This isn't an aging study. The control mice don't die because they are getting old -- they die early because they have been made sick by the diet. A more informative study would have compared SRT1720 to known drugs based on its ability to improve accepted metabolic parameters. I suspect that SRT1720 doesn't look so great in that comparison. I'm certain that paper wouldn't have been covered in the NY Times.

Of course, even if we conclude that the claims of increased "healthspan" for SRT1720 are correct, demonstrating the reversal of the unhealthy consequences of chronic obesity is not the same thing as preventing them from occuring in the first place. The road ahead still looks rocky to me even with the rosiest of spectacles.

When you see "dose pushing" to this level to see efficacy in a rodent model, then one MUST keep an open mind and consider off target effects. It's that simple. If your drug is optimized for a specific biologic target,you have sufficient plasma and target tissue drug exposure then one must think twice about the target. There are some instances that could be confusing: Competitive inhibitors of kinases where the Km for ATP is in the low micromolar range. Given that there are 5 mM levels of ATP in cells/in vivo, you will pull your hair out trying to show efficacy but you never will be able to given the swamping ATP effect in vivo. I've seen many programs go down due to lack of cellular activity and efficacy. Sometimes it's the target/enzyme that is the problem, one needs to keep in mind kinetics, expression, and whether or not the enzyme/target is in fact active or dormant in vivo (Ie: pH effects in the cell, swamping with substrate / agonist in vivo. In the case of Sirt1, I think one must look at the origin of the research.... Resveratrol,a polyphenol/stilbene derivative. Any in tune Med. chemist will say: off target effects, anti-oxidant effects possibly affecting oxidative stress in the cell etc....Starting a full blown med. chem. optimization of a polyphenol is high risk to say the leaset and one would think that BIG PHARMA has figured that one out. Based on my experience, they haven't because they don't listen to their scientists!!!!!!

I think I have exceeded my free access to NYT this month but the commentary Derek satirized sounds a lot like what I would have said: the motive of the treatment is to allow overweight mice and people to live longer. Same concept as airbags I guess but car accidents sometimes get caused by other drivers or lightening, or something outside the responsibility of the airbag plunger.

@5. If the animals don't like the taste of the food, they tend to kick it out of the dish. This can go unnoticed because it gets mixed with droppings on the bottom of the cage or gets kicked out of the cage altogether. If you don't actually see them doing it, its easy to just weigh the dish and assume the missing food was consumed.

@4 and @13,
Resveratrol is an amazing compound. You dip shits in big pharma who make all sorts of drugs with unpredictable and deleterious side effects, who pass the FDA with all sorts of arm twisting, you guys are the unsavory.
Resveratrol, at moderate doses, has been torpedoed by the powers that be. But, science will, and is, prevailing. You guys are dupes.

@14 John
I admire you for staying with your common sense arguments to make sure that everyone is aware of *the elephant in the cage* ;-)

@15 "You guys are dupes" [Mad Chemists?}
How so? As a Mad-But-Savvy Med Chemist, I will stick with a regular glass of red wine to get my therapeutic dose of polyphenols.
These *designer* active substances at toxic hit-multi-targets doses will probably never make it past studies in animals.
Let the Molecular Biology revolution roll.

Why would it be bad to find a drug that lets overweight people live a lot longer? Given the obesity epidemic in the US and Europe, and rising obesity levels in parts of Asia, wouldn't such a drug be a really good thing? (Or do the trolls assume that we have the ability to create magic weight-loss drugs and just choose not to develop them, along with the cancer cures we're supposedly hiding?)

@ dichotomous (22): What would your clinical endpoint be in a trial? If there's no weight loss and only long term hints of metabolic stabilization (at mega doses), then you're talking about one nightmare to develop. Do you just wait around to see how long it takes them to die? It's usually desirable to have your clinical endpoints read out a little faster than that.

BTW - there already IS a treatment that lets overweight people live longer that has been approved for a while now. It's very unpopular but it's called Diet and Exercise.

Also given our ability to synthesize life in chemistry labs (thanks to craig venter) and amazing advances in genetic engineering (dolly the sheep, transparent frogs), should not we move towards creating lab animals for testing new drugs , which have similar metabolism, dosage and toxicology profile like humans, so that these useless nematodes and confusing mice studies dont let us drain billions and resulting job cuts....

no apologies to PETA, i have much more regards for my own species ........

I wish these guys would learn about dose-ranging. Assuming the drug has decent bioavailabiity (I don't know that), then the only explanation for the giant doses is that the people designing the study want to see a "big" effect. Of course, the taste aversion hypothesis sounds quite reasonable to me, and if the animals in the treatment group did lose 10% of their body weight, that is quite significant. It is well known in the species everyone cares about (H. sapiens) that even a small drop in body weight has huge effects on inflammatory biomarkers.

@cynical1 (#23) - I don't disagree with any of your points; long-term mortality is tough to study if the goal is to make money, and diet and exercise are superior to a pill (especially if you believe there are no free lunches).

I'm reacting more to the shock and opprobrium given our industry for trying to create something that would, in theory, be medically really useful. Why wouldn't a drug that let people with bad diet and exercise live longer be a good thing? Isn't that what statins are? The only difference is that we have a nice biomarker for statin function, even though ENHANCE seemed to call its validity into question.

Allometric scaling across species has confused a lot of people recently (specifically in the case of resveratrol), and it appears even some industry veterans.
Check out the excellent paper from Nihal Ahmad et al., The FASEB Journal, 2008; 22, 659-661. (http://www.fasebj.org/content/22/3/659.full).
It deals with scaling across multiple species.
In short, the allometric dose translation from mice to humans would predict a human dose of 8.1 mg/Kg (total dose of 486mg for a 60Kg person).

Indeed, having worked in preclinical tox for a while, that dose of 100 mpk didn't strike me as particularly huge. In the upper range of what's used in preclinical studies, perhaps, but nowhere near the highest mpk doses i've seen. When dealing with oral dosing, it's all about absorption, bioavailability and activity - the actual mpk number isn't all that revealing.

Food intake control is vital in understanding lifespan effects in mice as it is very clear that lifespan correlates very well to food intake. Although the authors state there were no differences in caloric intake in the drug treated mice, if you quantify the caloric intake date in the Supplemental section, you'll see that there is a dose-dependent effect on caloric intake. The high dose SRT1720 group ate only 88% that of the control high fat diet group over the course of ca. 1 year. That, my friends, is a very significant difference from a metabolic perspective. In fact, if you graph median lifespan versus cumulative caloric intake across the 4 groups in the study you
get very near to a staight line (r2= 0.89). I conclude that the lifespan effects of SRT1720 in this study are largely driven by its effects on food intake. For details see: https://sites.google.com/site/srt1720lifespan/home

This SRT-compound's controversy powered by a cycle of publication and data mining, I believe, is a smoke screen to keep intelligent minds from discovering the real reason behind the 720M purchase. There is a reason, and its a good one.

Only a remnant of individuals will be able to come up with the scenario to which I am pointing to. For the others, look at the media - see how they control what you think about and argue about. What's really lingering behind that screen is hitting us all hard now - now that its too late.

What does Sirtris really have? Perhaps something that due to the work of a marine biologist in 1995
can't deliver them a composition of matter patent? A compound that is off the chart - excellent for RA and diabetes - but can't make money? No not Resveratrol - think harder.

Good luck and remember, in this decade, it's smoke screens and mirrors.